DecayInFlyTruthTrajectoryBuilder.cxx

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/*
  Copyright (C) 2002-2025 CERN for the benefit of the ATLAS collaboration
*/
 
// Andrei Gaponenko, 2008
 
#include "TrkTruthCreatorTools/DecayInFlyTruthTrajectoryBuilder.h"
 
#include "TrkTruthData/TruthTrajectory.h"
 
#include "AtlasHepMC/GenParticle.h"
#include "AtlasHepMC/GenVertex.h"
#include "TruthUtils/HepMCHelpers.h"
 
#include "GeneratorObjects/HepMcParticleLink.h"
#include "AthContainers/DataVector.h"
 
#include <stack>
#include <cmath>
namespace Trk {
 
//================================================================
DecayInFlyTruthTrajectoryBuilder::
DecayInFlyTruthTrajectoryBuilder(const std::string& type,
                  const std::string& name,
                  const IInterface* parent)
  :  AthAlgTool(type,name,parent)
{
  declareInterface<Trk::ITruthTrajectoryBuilder>(this);
}
 
 
//================================================================
StatusCode DecayInFlyTruthTrajectoryBuilder::initialize() {
  return StatusCode::SUCCESS;
}
 
//================================================================
void DecayInFlyTruthTrajectoryBuilder::
buildTruthTrajectory(TruthTrajectory *result, const HepMC::ConstGenParticlePtr& input) const
{
  result->clear();
  if (input) {
    HepMC::ConstGenParticlePtr  next{nullptr};
    HepMC::ConstGenParticlePtr  current = input;
 
    // Extend trajectory outwards.  The last particle should go at [0]
    // in the TruthTrajectory, so we need to use a tmp storage while
    // traversing the structure.
    std::stack<HepMC::ConstGenParticlePtr> tmp;
    while ( (next = getDaughter(current)) ) {
      tmp.push(current = next);
    }
 
    // All particles in the TruthTrajectory will be from the same GenEvent
    const int eventNumber = input->parent_event()->event_number();
 
    // copy the outer half to result
    while (!tmp.empty()) {
      result->emplace_back(tmp.top(), eventNumber, HepMcParticleLink::IS_EVENTNUM);
      tmp.pop();
    }
 
    // The input particle itself
    result->emplace_back(input, eventNumber, HepMcParticleLink::IS_EVENTNUM);
 
    // Now continue towards the interaction point
    while ( (next = getMother(current)) ) {
      result->emplace_back(current = next, eventNumber, HepMcParticleLink::IS_EVENTNUM);
    }
  }
}
 
//================================================================
DecayInFlyTruthTrajectoryBuilder::MotherDaughter
DecayInFlyTruthTrajectoryBuilder::truthTrajectoryCuts(const HepMC::ConstGenVertexPtr& vtx) 
{
  HepMC::ConstGenParticlePtr mother{nullptr};
  HepMC::ConstGenParticlePtr daughter{nullptr};
     // only truth vertices with 1 incoming particle     
      // Restrict to quasi-elastic processes (e.g. brems, delta-rays, pi->pi+Delta).
      // 
      // Require not more than 2 outgoing particles. Note that
      // delta-rays for primary==electron is a special case, because we have two
      // outgoing particles with the same PDG id.  The "correct" one
      // is that with the higher energy (NOT pt).
      // 
      // allow 1 outgoing to cover possible vertexes from interaction in detector material
  if(vtx && (vtx->particles_in_size() == 1) && (vtx->particles_out_size() <= 2) ) {
 
    mother = vtx->particles_in().front();
    // Allow status code 1 and 2.  E.g. a pion that produced a long track can decay  outside of InDet and have status==2.
    if( mother && MC::isPhysical(mother) ) {
 
    int num_passed_cuts = 0;
    HepMC::ConstGenParticlePtr passed_cuts{nullptr};
    for(const HepMC::ConstGenParticlePtr& candidate: *vtx){
      if(candidate->pdg_id() == mother->pdg_id()) {
 
        if(passed_cuts && MC::isElectron(mother)) { // second negative electron is a special case
          if(candidate->momentum().e() > passed_cuts->momentum().e()) {
        passed_cuts = candidate; // don't increment num_passed_cuts, we are replacing chosen particle
          }
        }
        else {
          passed_cuts = candidate;
          ++num_passed_cuts;
        }
      }
      // allow pi/k->mu decay
      else if(std::abs(candidate->pdg_id()) == 13){
        passed_cuts = candidate;
        ++num_passed_cuts;
      }
    }
 
    if(num_passed_cuts==1) { // disallow hadronic pi->N*pi etc.
      daughter = std::move(passed_cuts);
    }
    }
  }
  
  return std::make_pair(mother, daughter);
}
 
//================================================================
HepMC::ConstGenParticlePtr DecayInFlyTruthTrajectoryBuilder::getDaughter(const HepMC::ConstGenParticlePtr& mother) const {
  if(mother) {
    MotherDaughter res = truthTrajectoryCuts(mother->end_vertex());
    if(res.first == mother) return res.second;
  }
  return {nullptr};
}
 
//================================================================
HepMC::ConstGenParticlePtr DecayInFlyTruthTrajectoryBuilder::getMother(const HepMC::ConstGenParticlePtr& daughter) const {
  if(daughter) {
    MotherDaughter res = truthTrajectoryCuts(daughter->production_vertex());
    if(res.second == daughter) return res.first;
  }
  return {nullptr};
}
 
//================================================================
 
} // namespace Trk